Impedance networks are well known in the art and typically include one or more resistor structures which are combined so as to provide gain configurations to divide a voltage level into substantially proportionally matched levels. They are commonly used in amplifier configurations, digital-to-analog converters, analog-to-digital converters and indeed in widespread analog and RF circuitry applications. When implemented in an integrated circuit (IC) environment, the resistor is formed by the creation of a resistive element in a layer of the IC. The physical configuration of the resistive layer may vary depending on the application. Examples of such known configurations are shown in FIGS. 1A to 1D which show respectively a standard resistor 100, a dog-bone resistor 105, a wide resistor configuration 110 and a tabbed resistor 120. Although the physical layout of these configurations may vary, they all utilize end point contact layers 115 so as to provide an equivalent two terminal impedance. As shown in FIG. 1C, the end point contact layers 115 may be provided in one or more rows; in the embodiment of FIG. 1C, two rows of three contact layers are provided. The contact impedance provided by the physical contact of the end point contact layers to the base IC structure has a contributing effect towards the overall resistance of the structures, which is undesirable as it is not an accurately definable value and may drift during operation. The series contact structure has a non-zero impedance. The finite series impedance effect of the contact structure can be undesirable as it is different from the base layer of the resistor, and the contact structure often has less well controlled and lower performance resistive properties.
Examples of the use of resistor string configurations are described in U.S. Pat. No. 6,452,519, which provides for a placing of contacts to the resistor string outside of the current path of the resistor string so as to provide a resistor string having a very low temperature drift.
It is also known to provide what are commonly called “snake” or “ladder” configurations. Examples of these configurations are described in U.S. Pat. No. 5,268,651 and EP 0 955 678. In the configuration of U.S. Pat. No. 5,268,651 an integrated circuit resistor structure is described which has a forced high end and a forced low end and is designed for specific application for use in instrumentation amplifiers. It includes an operational amplifier which regulates the current between the force high connection and the force low connection in response to the voltage sensed in internal sensing connections of the resistor structure. This requirement for sensing elements requires an asymmetric resistor structure design and is specifically suited for high impedance sensing applications.
EP 0 995 678 describes the uses of a ladder type structure for application in digital to analog converters (DACs). The resistor structure provides a conducting path with a path meander configuration so as to provide a voltage divider network. At each location where a voltage level is to be established the conducting path has an expanded region, called a junction region. The centres of all the junction regions are equidistant from the centres of neighbouring junction regions, and have a metal patch extending therefrom. The metal patches are coupled to conducting plugs, or contacts, that can be coupled to switching elements of a DAC. Although the physical centers of the junction regions may be equidistant, the current path in the junction regions is not Manhattan, or perpendicular, in nature. This is somewhat shown by the voltage profile of FIG. 2 of the specification. This simulation output superimposed upon the structure does not appear to show the true real-life effect of the metal patch, or contact, in the centre of this region. Such contact structures in the current path are disadvantageous.
There is therefore a need to provide a snake-like network impedance structure that reduces the contact impedance contribution of the individual elements and the overall resistance of the structure.
Accordingly, there is a need for a resistor network configuration that is adapted to minimize the contribution of the contact impedances to the overall resistance of the structure.